Methyl ethers of branched monoolefins such as methyl-t-butyl ether are known additives to gasoline which improve the octane rating of the mixture. Such methyl ethers are produced by reacting methanol with isobutylene, or the corresponding higher branched monoolefin, in the presence of an acidic catalyst. The reaction can proceed with mixed hydrocarbon streams containing the branched monoolefin, with most catalytic systems being generally economically limited to hydrocarbon streams containing at least about 50% of the desired branched monoolefin, typically in combination with other monoolefins, diolefins and alkanes.
Among the common catalysts for the reaction are zeolites, acidic ion exchange resins and acidic inorganic materials. The reaction is normally conducted either in a batch fashion or with co-current feed of methanol and hydrocarbon. Unfortunately, the most common source of branched monoolefins, such as isobutylene, is in refinery operations wherein the isobutylene is present as less than 50% of a hydrocarbon stream, and is frequently also present with butadiene. While linear monoolefins and alkanes (e.g., butane) are inert in the presence of methanol and such acidic catalysts, the presence of butadiene complicates the reaction with most conventional catalysts, producing by-products such as polybutadienes.
U.S. Pat. No. 4,218,569 to Chase et al. (1980) discloses a process of providing methyl-t-butyl ether (MTBE) suitable for gasoline-blending and C-4 hydrocarbons depleted of isobutylene suitable for alkylation. This patent indicates that simple distillation will not produce a C-4 overhead free of methanol in amounts objectionable for alkylation; and therefore the patent recommends passing the distillate through a solvent such as ethylene glycol to lower its methanol content to below 100 ppm. Such a method requires three steps reaction, distillation and stripping--and the resultant complications and capital expense. If the methanol content (about 5 weight %) of the MTBE stream is to be lowered, a fourth step (methanol stripping of the MTBE) would be required. It would be desirable to produce depleted C-4 and MTBE streams both low in methanol directly.
U.S. Pat. No. 2,720,547 to Wolff et al. (1951) describes a process in which mixed butenes are fed adjacent to the bottom and a mixture of methanol and alkanesulfonic acids are fed adjacent the top of a reaction column operating at -50.degree. C. to +50.degree. C. An overhead stream containing unreacted butenes and a bottoms stream containing alkanesulfonic acid catalyst, methyl-t-butyl ether product and small amounts of methanol are both fed from the reaction column to a fractionating column operating at lower pressure, with excess methanol added to the bottoms stream. Four streams are removed from the fractionating column, one of which contains methyl-t-butyl ether and methanol as a azeotrope. Methanol is scrubbed from the methyl-t-butyl ether with water.